Why is needlescopic hernia repair not performed on adults?

Easy answer: adults are not larger children.
In children: Hernias are anatomical failures to close. The tissue itself is otherwise normal. If you close it surgically, it will heal.
In adults: Hernias are structurally abnormal tissues. They have poor collagen content. The tissue is weak. If you close it like in a child, it will reopen. It needs a mesh to facilitate its closure or strengthen the tissue. Or you need to close it with multiple layers of overlap and accept that a proportion of them may still fail and need mesh.